Generally, a stepping motor produces discrete angular movements of substantially uniform magnitude in response to electrical pulses rather than continuous rotation. It is known that stepping motors can have instabilities during changes in the driving frequency. German Patent DE No. 34 44 220 A1 (corresponding to U.S. Pat. Nos. 4,683,409 and 4,673,855) at FIG. 1 shows a driving device for a stepping motor used to compensate for instabilities in the stepping motor.
The driving device disclosed in the German patent includes a voltage controlled pulse generator for producing rectangular electrical pulses at a variable frequency. The electrical pulses are filtered and in one embodiment amplified to produce a voltage signal. This voltage signal has a source resistance based on an electrical current representing the sum of the phase electrical currents of the stepping motor. The pulse generator is an astable multivibrator including a timer of the type commercially available as a LM 556 timer. The multivibrator includes an RC network for controlling the time basis of the duration of each electrical pulse, pulse width, and the time period between consecutive electrical pulses, the pulse frequency.